Multi-layer, self-aligned vertical combdrive electrostatic actuators and fabrication methods

ABSTRACT

A method of fabricating multi-layer vertical comb-drive actuator that includes a first comb structure having a plurality of first comb fingers and a second comb structure having a plurality of second comb fingers, wherein the first and second comb fingers are substantially interdigitated. The present invention includes masking and etching of a structure that contains these multiple layers, wherein the first and second comb fingers are simultaneously fabricated. The first and second comb fingers may include two or more stacked conductive layers electrically isolated from each other by an insulating layer or an air gap. Alternatively, either the first or second comb fingers may include only one conductive layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Provisional application 60/192,097filed Mar. 24, 2000, which is herein incorporated by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to micro-electromechanicalsystems (MEMS). More particularly, it relates to vertical comb-driveelectrostatic actuators and fabrication methods.

BACKGROUND ART

[0003] Microstructures fabricated using silicon integrated processingtechniques are used in a wide variety of sensing, actuating, and opticalapplications. One particularly useful device is a comb-drive actuator,which consists of two comb-like structures, one mobile and onestationary, whose fingers are interdigitated. When a potentialdifference is applied to the alternating fingers, a resultingelectrostatic force causes the mobile fingers to move to maximize theoverlap area. While the force provided by each finger is quite small,including a large number of fingers in the comb drive allows forapplication of relatively large forces using low voltages, particularlywhen there is a large capacitive overlap area between two adjacentfingers. Comb drives also provide a method for accurate positionmeasurement by sensing of the capacitance of the fingers.

[0004] Comb drives are differentiated by the plane of motion of thestationary and mobile combs with respect to one another. Linear orlateral comb-drive actuators provide translational motion in a singleplane as the two comb devices move from being relatively spaced apart tobeing fully interdigitated. The two comb structures remain in the sameplane during actuation, with the stationary comb being fixed to asubstrate, and the mobile comb moving with respect to the substrate.Examples of lateral comb drives are disclosed in U.S. Pat. Nos.5,025,346, issued to Tang et al., and 5,998,906, issued to Jerman et al.

[0005] It is often desirable to create out-of-plane actuation of variousmicrostructures, such as rotation of mirrors about an axis parallel to asubstrate. These rotating mirrors can be used individually or in arrayform for applications such as adaptive optics, visual displays, orfiber-optic switching. Vertical comb-drive actuators provide rotationalmotion or translational motion perpendicular to a substrate. Amicromachined electrostatic vertical actuator is disclosed in U.S. Pat.No. 5,969,848, issued to Lee et al. The device of Lee et al. contains aset of vertical comb drives, with each drive capable of deflecting oneedge of a square mirror. By relying on an asymmetric distribution ofelectrical fields when a bias voltage is applied between stationary andmovable comb fingers, the device of Lee et al. allows a small vertical(i.e. out of the plane of the comb fingers) motion of each mirror edge,at most 1.5 μm.

[0006] Larger movements and more simplified fabrication techniques areprovided by staggered vertical comb drives, in which the stationary andmoving comb drives are positioned parallel to one another, but with theplane of the moving comb above the plane of the stationary comb. Thestationary comb fingers are an integral part of the substrate, while themoving comb is fixed to the substrate only through flexures. Applying avoltage between the two comb layers causes the moving comb teeth to beattracted to the stationary teeth and move to increase the overlap area,thereby exerting a force on the moving comb. Conventional fabricationtechniques for vertical comb drives using standard photolithographyprocesses require multiple steps for patterning the comb fingers. First,one set of comb teeth is fabricated on a first wafer layer. A secondwafer layer is then bonded on top of the first wafer layer, followed bypatterning and etching of a second layer to form the second set of combteeth. The two wafer layers must be aligned to a very high precision;typical applications require comb fingers of 2 μm wide with a 6 μmseparation distance, so that adjacent overlapped fingers are separatedby only 2 μm. Fabrication of vertical comb drives using this techniqueis prone to alignment problems. The steppers used to align theindividual die on a wafer typically have a lateral resolution of ±0.25μm. This resolution places a lower limit on the gap between adjacentcomb fingers of about 2 μm. Because two adjacent fingers are atdifferent potentials during operation, they cannot contact each other.At high actuation voltages, errors in alignment of the fingers can causesideways motion and instability in the comb drive. As a result,conventional fabrication techniques typically have low productionyields.

[0007] There is a need, therefore, for a vertical comb drive that can befabricated in fewer steps than required by conventional fabricationmethods, and that provides accurate alignment between two layers of combfingers without requiring complicated alignment procedures.

SUMMARY

[0008] The present invention provides a multi-layer vertical comb driveactuator in which first and second comb fingers are simultaneouslyfabricated from a single multi-layer substrate. Because the fingers arefabricated together, the tedious alignment of the first and secondfingers, required for fabricating conventional vertical comb-driveactuators, is avoided. Alignment is a direct result of the mask used infabrication; thus the device is referred to as self-aligned. Each fingerhas two vertical conductive layers separated by an insulating layer oran air gap, and movement is provided by attraction of opposite layers ofthe first and second comb fingers.

[0009] The present invention provides a multi-layer vertical comb driveactuator containing a first comb structure having a plurality of firstcomb fingers, and a second comb structure having a plurality of secondcomb fingers. The second comb fingers extend from a comb bridgeconnected to the substrate through one or more flexures allowingvertical movement or rotational movement about an axis, and arepositioned to be interdigitated with the first comb fingers. A movableelement is attached to the rotatable flexure and coupled to the secondcomb structure. In one embodiment, both the first comb fingers and thesecond comb fingers may include first and second conductive layerselectrically isolated from each other by an insulating layer or air gap.The first conductive layers of the first comb fingers may besubstantially aligned with the first conductive layers of the secondcomb fingers, and the second conductive layers of the first comb fingersmay be substantially aligned with the second conductive layers of thesecond comb fingers. In an alternate embodiment, the second comb fingersmay have only a first conductive layer in vertical alignment with thefirst conductive layer of the first comb fingers. In a furtheralternative embodiment, the second comb fingers have first and secondconductive layers electrically isolated from each other by an insulatinglayer or air gap, and the first comb fingers have only a firstconductive layer in alignment with the first conductive layer of thesecond comb fingers. In all embodiments, applying a voltage between thefirst and second layers of both first and second fingers causes thesecond comb structure to deflect, thereby maximizing the overlap areabetween the opposite layers of the first and second comb fingers. Thisvertical motion can be used to cause rotation if the movable element ismounted with a rotational degree of freedom.

[0010] Preferably, the actuator also has a position sensor for measuringthe position of the movable element, and such position telemetry is fedinto a feedback mechanism coupled to the voltage source for controllingthe position of the moveable element. Combdrive fingers can also performa position sensing function in addition to driving the angular rotationof the movable element attached thereto, by reading the capacitance ofthe fingers, indicating a position of the movable element. This positionsensor embodiment may include capacitive sensing between anycombinations of the comb layers. Alternatively, other position sensors,such as gap closing electrodes, additional comb fingers, piezoresistivestrain gauges, coils, magnets, piezoelectric sensors and opticalsensors, can be used to track the position of the movable element by oneskilled in the art.

[0011] The actuator may have a feedback mechanism coupled to theposition sensor and the voltage source for controlling the position ofthe movable element. The various position sensors may be used in tandemto increase the position tracking accuracy of the sensor. Furthermore, Afirst sensor can be linked to a second position sensor to configure ortune the second sensor enabling better accuracy position tracking thanotherwise provided by two unlinked sensors.

[0012] Actuators of the present invention may be one-dimensional ortwo-dimensional gimbaled actuators. In a two-dimensional actuator therotatable flexure may be attached to a frame, which may be mechanicallyengaged with a second rotatable flexure attached to a substrate anddisposed along a second axis. The frame may also be coupled with afourth comb structure that may have a plurality of fourth comb fingersthat may be substantially co-planar with a plurality of third combfingers extending from a third comb structure. Either or both of thethird and fourth comb fingers may include first and second conductivelayers electrically isolated from each other by an insulating layer, anair gap or by any means as one skilled in the art would be capable ofapplying. Third comb fingers and fourth comb fingers may besubstantially interdigitated in a second predetermined engagement. Thesecond axis may be substantially orthogonal to the first axis in thisembodiment. Two independent voltages may be applied to control rotationof the actuator in two orthogonal first and second axes.

[0013] Also provided is a method of fabricating the differentembodiments of the multi-layer vertical comb-drive actuator of thepresent invention. The method contains the following steps: providing amulti-layer structure having first and second conductive layersseparated by an insulating layer, and etching a top pattern in the firstand second conductive layers and insulating layer to define the secondand first comb fingers. The substrate may also have additional layersthat are etched to define the bottom surfaces of the fingers.

[0014] In an alternate embodiment of the fabricating method, the firstconductive layer may be removed from either the second or the firstfingers in an additional step, to leave only the second conductivelayer.

BRIEF DESCRIPTION OF THE FIGURES

[0015] FIGS. 1A-1C are simplified schematic diagrams of multi-layercomb-drive actuators according to a first embodiment of the presentinvention.

[0016]FIG. 1D is a simplified plan view of a two-dimensional rotatingactuator according to an alternative embodiment of the presentinvention.

[0017] FIGS. 2A-2E are simplified cross-sectional views showingfabrication of a multi-layer comb-drive actuator according to a secondembodiment of the present invention.

[0018] FIGS. 3A-3I are simplified cross-sectional views showingfabrication of a multi-layer comb-drive actuator according to a thirdembodiment of the present invention.

DETAILED DESCRIPTION

[0019] Although the following detailed description contains manyspecifics for the purposes of illustration, anyone of ordinary skill inthe art will appreciate that many variations and alterations to thefollowing details are within the scope of the invention. Accordingly,the following embodiments of the invention are set forth without anyloss of generality to, and without imposing limitations upon, theclaimed invention.

[0020] The present invention provides a multi-layer vertical comb-driveactuator. Rather than being in different planes, the second comb fingersand first comb fingers lie in the same plane, each having first andsecond conductive layers separated by an insulating material, layer orair gap. The opposite layers of the second and first structures may beattracted to each other when voltage is applied between opposite layersof the first and second comb fingers, thus providing vertical and/orrotational motion.

[0021] A preferred embodiment of a multi-layer vertical comb-driveactuator 10 of the present invention is shown in FIG. 1A. The actuator10 is formed on a substrate 12. A first comb structure 25, may beattached to the substrate 12 contain first comb fingers 14 that may havefirst conductive layers 16 and second conductive layers 18, which may beelectrically isolated from each other by a first insulating layer 20. Asecond comb structure 22 may contain second comb fingers 24 that mayextend from a comb bridge 26. The first comb fingers 14 mayinterdigitate with the second comb fingers 24. By way of example, thewidth of comb fingers 14 and 24 may be approximately 6 μm, with aseparation distance between adjacent fingers of approximately 2 μm. Thefirst comb structure 25 may be electrically isolated from the secondcomb structure 22 and/or the substrate 12.

[0022] In the embodiment shown in FIG. 1A, the second comb fingers 24and comb bridge 26 may have first conductive layers 30 and secondconductive layers 28 electrically isolated from each other by aninsulating layer 32. The insulating layers 20, 32 of the first andsecond comb fingers 14, 24 may include layers of insulating materials,such as silicon oxide or an insulating air gap. The first and secondconductive layers 28, 30 of the second comb fingers 24 may besubstantially aligned with the corresponding first and second conductivelayers 16, 18 of the first comb fingers 14.

[0023] A movable element 36 is mechanically coupled to the second combstructure 22 and the substrate 12 by a flexure 34. The flexure 34 may bea rotatable flexure that allows the movable element 36 to rotate aboutan axis 38. Such a rotatable flexure may be any structure suitable forproviding a torque that counters rotation of the movable element 36about the axis 38, such as one or more torsion hinges, cantileverflexures, serpentine flexures, or pin-and-staple hinges combined withone or more springs. The flexure 34 may also be a flexible member thatallows vertical movement of the movable element with respect to theplane of the substrate 12. Alternatively, the torque that counters themovement of the moveable element 36 can be provided non-mechanically ina controlled, fixed and variable mode by application of e.g. magnetic orelectrical forces onto the moveable element, or by controllably couplingthe torque through piezoelectric strain gauge. Non-mechanical torque isuseful to provide torsion force, for example, when using a pin andstaple hinge flexure that otherwise would not provide a restoring forcedirected to the movable element

[0024] Operation of actuator 10 may be configured to share similarity tothe operation of a conventional vertical comb-drive actuator. In onemode of operation, a voltage source 15 may apply a voltage between thefirst conductive layers 16, 30 and the second conductive layers 18, 28respectively of the first and second comb finger 14, 24. In theembodiment depicted in FIG. 1A, the first conductive layers 16, 30 ofthe first and second comb fingers 14, 24 may be grounded and the voltagesource 15 applies a voltage to the second conductive layers 18, 28 ofthe first and second comb fingers 14, 24. As a result of the appliedpotential difference, the first conductive layers 30 of the second combfingers 24 are attracted to the second conductive layers 18 of the firstcomb finger 14. The attraction causes the second comb structure 22 tomove relative to the first comb structure 25, which, in turn, causes themovable element 36 to rotate about the axis 38.

[0025] Although FIG. 1A depicts a voltage applied to the secondconductive layers 18, 28, while the first conductive layers 16, 30 aregrounded, the invention is in no way limited to this particularconfiguration for applying a voltage between the first and secondlayers. The second conductive layers 18,28 may be grounded while thevoltage source 15 applies voltage to the first conductive layers 16, 30.Alternatively, the voltage source 15 may apply voltages of oppositepolarity to the first and second conductive layers of the first andsecond comb fingers 14, 24. Voltage applied between the first and secondcomb fingers can be passed through various wave shaping schemes tooptimize control of the movable element. Other methods of applyingvoltage between the first and second comb fingers are well known tothose of average skill in the art.

[0026] A typical method of actuating the actuator 10 of FIG. 1A is toapply a voltage between the second conductive layers 18 and 28 of thesecond 24 and first 14 comb fingers and the first conductive layers 30and 16 of the second 24 and first 14 comb fingers. In thisconfiguration, the second comb finger 24 is in a state of unstableequilibrium in its nominal sate, and can rotate either upwards ordownwards, since for any given voltage, there are two stable states. Inone of the stable states, the first conductive portion 30 of the secondcomb fingers 24 may be engaged with the second conductive portion 18 ofthe first comb fingers 14, while in the other of the two stable states,the second conductive portion 28 of the second comb fingers 24 isengaged with the first conductive portion 16 of the static comb fingers14.

[0027] To alleviate this ambiguity, a more complex actuation scheme canbe employed. Such a scheme requires the use of more than two voltages onthe four conductive portions of the comb fingers. For example, the firstconductive layers 16 and 30 of both the first 14 and second 24 combfinger can be kept at ground, while the second conductive layer 18 ofthe first comb 14 can be kept at V, and the second conductive layer 28of the second comb 24 at V+dV, where dV is some additional voltage. Thiscauses the second comb structure 22 to move down relative to the staticcomb structure 25. The actuation scheme can be quickly turned back tothe original dual-voltage scheme once a preferred direction isestablished. Also, this scheme can be reversed to move the second combfingers upward. Similar actuation schemes employing different voltagesmay also be employed to give the actuator a preferred direction ofmotion.

[0028] A major advantage of the multi-level vertical comb drive of thepresent invention is the ability to have push-pull actuation. Voltagedifferences on neighboring electrodes can only generate attractiveforces. In conventional comb-drive actuators, a comb drive is used topull an actuator in one direction, and a mechanical spring force or anopposing comb drive oriented opposite the first are used to pull it inthe opposite direction. The present invention is unique in allowing asingle set of comb fingers to pull the second structure in oppositedirections, or in other words, to both pull and push the secondstructure. Push-pull actuation allows for a greater freedom to alter keysystem parameters using feedback schemes. Push-pull schemes allow boththe damping ratio and natural frequency of the system to be varied,while pull schemes only allow for the damping ratio to be varied. In theprior art vertical comb drives, only pull actuation methods areavailable. The pull actuation method of actuator 10 of FIG. 1A isdescribed above. An example of push-pull actuation follows. The actuatorcan be brought to a certain position by application of a voltage to thesecond conductive portions 18 and 28 of both the first 14 and second 24comb fingers, while keeping the first conductive layers 16 and 30 atground. To push the actuator back to its rest position with a largerforce than provided by the flexures, the polarity of the voltages on thetwo layers of either the first comb fingers 14 or the second combfingers 24 can be reversed. The dynamics of the movable element can bealtered by the application of simultaneous push-pull forces. Theactuator 10 may also be provided with a position sensor sense elementfor measuring a capacitance between the first and second comb fingers.For example, as shown in FIG. 1A, a sense element 17 may be coupled to afeedback element 19 that is coupled to the voltage source 15. Theposition sensor sense element 17 and feedback element 19 may beimplemented in hardware, software or some combination of both such as anapplication specific integrated circuit (ASIC). Many sensing andfeedback schemes are possible. For example, the sense element 17 maymeasure an amount of charge transferred to or from the comb fingers inresponse to the voltage applied by the voltage source 15. Alternatively,the position sense element 17 may apply a high frequency dither toeither first or second comb fingers. The position sensor sense element17 may then sense a return signal at the comb fingers not driven. Aphase difference between the dither signal and return signal determinesthe capacitance. Such capacitance can be correlated with the relativepositions of the second and first comb drives to obtain a very preciseposition measurement.

[0029] The position measurement may then be fed back to the voltagesource 15 via the feedback element 19 to control the relative positionof the movable element 36.

[0030] A differential capacitance measurement method may also beemployed. This allows for greater sensitivity and intolerance toenvironmental variations such as temperature. In a differentialcapacitance sensing scheme, multiple capacitances are sensed, and theposition may be calculated by using these multiple capacitivemeasurements. For example, sense element 17 of FIG. 1A could sense thecapacitance between the first conductive layer 30 of the second combfingers 24 and the first conductive layer 16 of the first comb fingers14, and comparing it with the capacitance between the first conductivelayer 30 of the second comb fingers 14 and the second conductive layer18 of the first comb fingers 14. Alternatively, a single capacitance maybe sensed between any two electrically isolated layers, e.g., layers 30and 16, layers 18 and 28, layers 16 and 28, or layers 18 and 30. Similarmethods may be employed with the structures in FIGS. 1B and 1C.

[0031] The present invention accommodates alternative position sensorscomprised of gap closing electrodes, additional comb fingers,piezoresistive strain gauges, coils, magnets, piezoelectric sensors,optical sensors and combinations thereof.

[0032]FIG. 1B shows an alternate embodiment of a multi-layer comb-driveactuator 40 of the present invention. Actuator 40 may share similarityto actuator 10. The actuator 40 may generally include a first combstructure 25′ mechanically coupled to a substrate 12′ and a second combstructure 41 attached to a movable element 36′. The movable element 36′may be mechanically coupled to the substrate by one or more flexures34′, e.g., for rotation about an axis 38′. The second comb structure 41may include second comb fingers 42 that extend from a comb bridge 46.One or more second comb fingers 42 of the second comb structure 41 haveonly a first conductive layer 44, and do not have second conductivelayers or insulating layers.

[0033] The first comb structure 25′ may have first comb fingers 14′ thatinterdigitate with the second comb fingers 42. The first comb fingersmay include first and second conductive layers 16′, 18′ electricallyisolated by an insulating layer 20′ or by an air gap. The firstconductive layer 16′ of the first comb fingers 14′ may be substantiallyaligned with the first conductive layer 44 of the second comb fingers.Alternatively, the second conductive layer 18′ of the first comb fingers14′ may be substantially aligned with the first conductive layer 44 ofthe second comb fingers.

[0034] Actuator 40 may be configured to operate similar to that ofactuator 10. For example, a voltage source 15′ may apply a voltagedifference between the conductive layer 44 of the second comb fingers 42and either of the conductive layers 16′, 18′ of the first comb fingers14′. The voltage source 15′ may also be coupled to a sense element 17′and feedback element 19′ as described above.

[0035]FIG. 1C shows a further alternate embodiment of a multi-layercomb-drive actuator 50 that generally includes a first comb structure 51mechanically coupled to a substrate 12″ and a second comb structure 22″attached to a movable element 36″. The movable element 36″ may bemechanically coupled to the substrate by one or more flexures 34″, e.g.,for rotation about an axis 38″. The second comb structure 22″ may havesecond comb fingers 24″ that extend from a comb bridge 26″. The combbridge 26″ and second comb fingers 24″ may have first and secondconductive layers, 28″, 30″ electrically isolated by an insulating layer32″. The first comb structure 51 may be similar to the first combstructure 25 of FIG. 1A except that one or more first comb fingers 52 ofthe second comb structure 51 may have only first conductive layers 54,and may not have second conductive layers or insulating layers. Thefirst conductive layer 54 of the first comb fingers 52 may besubstantially aligned with the first conductive layer 30″ of the secondcomb fingers 24″. Alternatively, the first conductive layer 54 of thefirst comb fingers 52 may be substantially aligned with the secondconductive layer 28″ of the second comb fingers.

[0036] Actuator 50 can be configured to operate similar to that ofactuator 10 and/or actuator 40. For example, a voltage source 15″ mayapply a voltage difference between the conductive layer 54 of the combfingers 52 and either of the conducting layers 28″, 30″ of the secondcomb structure 22″. The voltage source 15″ may also be coupled to asense element 17″ and feedback element 19″ as described above. Thedynamics of the movable element can be altered by the application ofsimultaneous push-pull forces.

[0037] When actuators 10, 40, or 50 are used for positioning a movableelement, such as a micromirror, the mirror may be formed integrally withthe second comb structure. Rotation of the second comb structure maycause the mirror to tilt, and the rotational flexures 34, 34″, 34″ mayprovide a restoring torque. The actuation mechanism may be integratedwith the mirror during manufacture, and may be linear and stable overquite a large range of motion. A relatively large torque allowsactuation at high speed, and enables large-angle steady-state beampositioning. Furthermore, the integrated device allows for capacitiveposition sensing. Thus the integrated device provides significantadvantages over existing magnetic, piezoelectric, and gap-closingactuators. However, The existing stated actuators can be used incoordination with the present invention to modify the dynamiccharacteristics of the movable element as desired by the application.

[0038] The present invention further provide a two-dimensional rotatingactuator including two multi-layer vertical comb-drives of the typesdepicted in FIGS. 1A-C, which are arranged in a gimbaled structure and arotating element mechanically coupled to both of the comb-drives asshown in FIG. 1D according to an alternative embodiment of the presentinvention. As shown in FIG. 1D, in a two-dimensional rotating actuator100, a second comb structure 118 having a plurality of second combfingers 117 that substantially interdigitate with a plurality of firstcomb fingers 115 extending from a first comb structure 116. Either orboth of the first and second comb fingers 115 and 117 may include twoconductive layers separated by an insulating layer or an air gap asdescribed in FIGS. 1A-C. First comb fingers 115 and second comb fingers117 may be substantially interdigitated in a first predeterminedengagement. Note that in the embodiment of FIG. 1D, first comb fingers115 are electrically isolated from second comb fingers 117, thesubstrate 102, frame 104 and rotating element 106.

[0039] The rotating element 106 may include a reflecting surface so thatthe device 100 may operate as a MEMS mirror. The rotating element 106may be attached to a first rotatable flexure 108 disposed along a firstaxis 124. Rotating element 106 may also be mechanically engaged withsecond comb structure 118 along with first movable comb fingers 117.First rotatable flexure 108 may be attached to a frame 104, which inturn may be mechanically engaged with a second rotatable flexure 110attached to a substrate 102 and disposed along a second axis 126. Frame104 may also be coupled with a fourth comb structure 114 having aplurality of fourth comb fingers 113 that substantially interdigitatewith a plurality of third comb fingers 111 extending from a third combstructure 112. The second comb structure 112 may be electricallyisolated from the substrate 102, the frame 104, the rotating element106, and the fourth comb fingers 113. Either or both of the third andfourth comb fingers 111 and 113 may include two conductive layersseparated by an insulating layer or an air gap as described in FIGS.1A-C. Third comb fingers 111 and fourth comb fingers 113 aresubstantially interdigitated in a second predetermined engagement. Thirdcomb fingers 111 are likewise electrically isolated from fourth combfingers 113. Moreover, first comb fingers 115 may be made to beelectrically isolated from third comb fingers 111. As such, the firstand second comb-drives are coupled by way of frame 104. First axis 124is configured to be substantially orthogonal to second axis 126 in thisembodiment.

[0040] It should be noted that first and second rotatable flexures 108,110, frame 104, rotating element 106, together with the first and secondcomb-drives, may be substantially co-planar. Furthermore, the rotatableflexures 108, 110 may be any structure suitable for providing a torquethat counters rotation of the second comb fingers about the first axis124, such as one or more torsion hinges, cantilever flexures, serpentineflexures or pin-and-staple hinges combined with one or more springs.Non-mechanical torque can be dynamically provided through other statedprincipals, including magnetic principles, and that the telemetrysensing of a first flexure may be linked to dynamically configure asecond torque element to achieve higher accuracy torsion control thanthe two unlinked elements could otherwise provide.

[0041] Applying a voltage from a source 120 between the second combfingers 117 and the first comb fingers 115 attracts the second combfingers 117 to the first comb fingers 115, which causes the first combstructure 118 to move vertically relative to the second comb structure116. Thus a torque is exerted on the rotating element 106, which causesthe rotating element 106 to rotate about the first axis 124. Applyinganother voltage from another source 122 between the fourth comb fingers113 and the third comb fingers 111 attracts the fourth comb fingers 113to the third comb fingers 111, which causes the fourth comb structure114 to move vertically relative to the third comb structure 112. Thus, atorque is exerted on the frame 104, which causes the frame 104 to rotateabout the second axis 126. Therefore the rotating element 106 can rotateabout the second axis 126. The applied voltages from the sources 120 and122 are typically about 30 V. The applied voltages from the sources 120and 122 may be adjusted to independently control the angle between theframe 104 and the substrate 102, and the angle between the rotatingelement 106 and the frame 104.

[0042] As described above, the capacitance of the vertical comb-drivesgenerally can be measured to monitor the angular positions of therotating element 106 and the frame 104. Furthermore, capacitance acrossthe comb fingers 115 and 117, or 111 and 113 may be used to sense theangular position of the rotating element 106. For example capacitancesensors 132 and 134 may be coupled to the comb fingers 111 and 115respectively. The capacitance sensors 132, 134 may provide feed back tothe voltage sources 120, 122 via controllers 136, 138. Capacitiveposition monitoring signals from the sensors 132, 134 may be used toimplement closed-loop feedback control the angles of the rotatingelement 106 and the frame 104 via the sensors 136, 138. Therefore,capacitive angle signals may be used in a servo loop to actively controlthe position of the rotating element 106. Several alternate positionsensing techniques, such as gap closing electrodes, additional combfingers, piezoresistive strain gauges, coils, magnets, piezoelectricsensors, optical sensors and combinations thereof could be used insteadof capacitive sensing from the comb fingers, and in tandem with thecapacitive sensing feature of the present invention. Furthermore, afirst position sensor element could be linked to configure a secondposition sensor element to achieve higher accuracy position sensing thanotherwise provided by two unlinked sensors.

[0043] The present invention also provides significant advantages overexisting vertical comb-drive actuators in its preferred fabricationmethod. Because the second comb fingers and first comb fingers are inthe same vertical plane, they can be formed in a single step from asingle multi-layer structure, providing for automatic alignment of thefingers. A method of fabricating a multi-layer vertical comb-drivestructure such as that depicted in FIG. 1A is illustrated in FIGS. 2A-2Eaccording to a second embodiment of the present invention. For asilicon-on-insulator (SOI) substrate, all of the steps can be performedusing standard photolithography tools, an oxide etcher, and a deepreactive-ion silicon etcher, all of which are available in commercialfabrication foundries. The following describes exemplary methods forfabricating comb-drive actuators of the present invention. It is to beunderstood that other fabrication methods may be used to make suchstructures, and that structures made by other methods are within thescope of the present invention.

[0044]FIG. 2A shows a cross-sectional view of a multi-layer substrate200 containing a first conductive layer 202, a second conductive layer204, a first insulating layer 208, a optional second insulating layer210, and an optional substrate layer 206. Conductive layers 202 and 204and the substrate layer 206 may be made of any suitable materialincluding, but not limited to silicon, silicon-germanium,silicon-carbide, nickel, and gold. Conductive layers 202 and 204 arepreferably silicon. Insulating layers 208 and 210 may be made of anysuitable insulating material including, but not limited to,silicon-nitride, silicon-oxide, silicon-carbide, quartz, highresistivity silicon, high resistivity silicon germanium, polyimide, or apolymeric film. Insulating layers 208 and 210 are preferably a siliconoxide formed by thermal oxidation of silicon conductive layers 202 and204, which are then bonded together. The substrate layer 206 ispreferably also silicon. Other substrates and combinations of materialsmay also be used in different fabrication processes.

[0045] In FIG. 2B, a masking layer 212 (for example, photoresist oraluminum) is deposited and patterned on top of layer 202 defining aplurality of first and second comb fingers 214, 216 respectively asshown in FIG. 2C. Note that the pattern is not uniform in a directionperpendicular to the page, but rather forms a comb structure similar tothose shown in FIGS. 1A-1C. FIG. 2C shows the comb fingers formed as aresult of deep reactive-ion etching of first and second conductivelayers 202 and 204 (e.g. silicon) and first insulating layer 208 (e.g.silicon oxide) of FIG. 2B, after removal of the remaining masking layer212. In FIG. 2D, a portion of optional substrate layer 206 (e.g.silicon) is etched away to release a bottom surface of the comb fingers214, 216, leaving first comb fingers 214 and second comb fingers 216.Second insulating layer 210 is also etched away either by anisotropicetching from the bottom side or by isotropic etching. Optional substratelayer 206 may instead be left intact, limiting motion of the second combfingers 216 to only motion above the substrate. Note that second combfingers 216 are connected to a second comb bridge in a plane parallel tothe plane of the paper, either above or below the paper. The firstinsulating layer 208 is optionally removed. Operation of the resultingactuator 220 is shown in FIG. 2E. A potential is applied to the toplayers of both the second and first (or stationary) comb fingers, whilethe bottom layers of both types of comb fingers are grounded. Thepotential can cause the second combs fingers 216 to move as shown.

[0046] Note that in the process illustrated in FIGS. 2A-2E, all of thecomb fingers, both first and second, are formed in a single etch. Thusthe proper alignment of the fingers is a result of precise fabricationof a single mask, which is easily attainable using standard techniques.Furthermore, it is possible to the reverse the order of the frontsideand backside etch steps, e.g., by selectively etching away portions ofthe substrate 206 before etching the conductive layers 202, 204 andinsulating layer 208 to define the comb fingers 214, 216.

[0047] A third embodiment of the present invention is illustrated inFIGS. 3A-3I, which shows a method of fabricating a multi-layer verticalcomb-drive structure of the type shown in FIGS. 1B-1C. In this method,one set of comb fingers has two layers, while the other has only onelayer. FIG. 3A shows a structure 300 containing first conductive layer302 and second conductive layer 304, and first and second insulatinglayers 306 and 308, respectively. Also shown is optional substrate layer310. Structure 300 may be identical to structure 200 of FIG. 2A.Conductive layers 302 and 304 are preferably silicon, while insulatinglayers 306 and 308 are preferably a silicon oxide formed from thermaloxidation of silicon wafer layers 302 and 304, which are then bondedtogether.

[0048] In FIG. 3B, a first masking layer 312 (e.g. silicon oxide,aluminum, photoresist) is deposited and patterned on top of firstconductive layer 302. Some of the remaining portions of the maskinglayer cover areas that will eventually become the first comb fingers.Next, in FIG. 3C, a second masking layer 314 (e.g. photoresist) isdeposited on top of first masking layer 312 and then removed accordingto a second pattern, defining the location of eventual comb fingers.Layers 312 and 314 contain different types of masking material, so thatone can be selectively removed without affecting the other. In FIG. 3D,regions 316 of first masking layer 312 that are not covered by secondmasking layer 314 are removed. This ensures that the second mask 314defines the comb structures. Therefore, the alignment between the firstmask 312 and the second mask 314 does not affect the comb widths. Next,in FIG. 3E, first conductive layer 302, first insulating layer 316, andsecond conductive layer 304 are etched, e.g., using deep reactive-ionetching (DRIE) to create two sets of comb fingers 322, 324 that willrespectively become second and first comb fingers. The second maskinglayer 314 is then removed to create the structure of FIG. 3F, which isetched using DRIE or other anisotropic silicon etching methods to removethe first conductive layer 302 from alternating comb fingers. Theresulting structure is shown in FIG. 3G. The fingers are then undercutin FIG. 3H, followed by optional removal of portions of the first andsecond insulating layers 306 and 308 and remaining first masking layer312 to reveal an actuator 330 of FIG. 3I. Insulating oxide layers may beremoved using a timed HF etch. In the embodiment shown, second combfingers 322 are connected to a second comb bridge in a plane parallel tothe plane of the paper, either above or below the page. The second combbridge may be connected to the substrate through a torsion hinge orflexure that allows movement of the second comb structure. One method tooperate actuator 330 is to apply a voltage V to the second comb fingers322 and a bottom layer 324B of the first comb fingers 324, while a toplayer 324A of first comb fingers 324 is grounded, causing an electricforce that moves the second comb fingers 322.

[0049] Note that the method illustrated in FIGS. 3A-3I can also be usedto create an actuator in which the first comb fingers have a singlelayer and the second fingers have two layers, in which case applicationof a voltage causes the second fingers to rotate downward. This methodrequires slightly different patterning of the two types of maskinglayers.

[0050] Actuators of the present invention may be used for any suitableapplication. Two-dimensional actuators may be fabricated using similarprocesses. Depending on the application needed, additional steps may beadded into the fabrication process to create an integrated device.Metals may be evaporated, sputtered, or electroplated onto the substrateusing methods known in the art.

[0051] In both embodiments of the fabrication method shown, all of thefingers are formed in a single process in a single multi-layer waferstructure, thus providing for very high precision in alignment of thecomb fingers.

[0052] It will be clear to one skilled in the art that the aboveembodiment may be altered in many ways without departing from the scopeof the invention. Accordingly, the scope of the invention should bedetermined by the following claims and their legal equivalents.

What is claimed is:
 1. A method of fabricating a multi-layer verticalcomb-drive structure comprising: a) providing a multi-layer structure,the multi-layer structure including: ii) a first conductive layer; iii)a second conductive layer; iv) a first insulating layer disposed betweenthe first conductive layer and the second conductive layer; and b)etching a pattern in the first conductive layer, the insulating layer,and the second conductive layer, wherein the pattern defines a pluralityof comb fingers of a first comb structure that interdigitate with aplurality of comb fingers of a second comb structure.
 2. The method ofclaim 1 , wherein the multi-layer structure includes a second insulatinglayer disposed between the second conductive layer and a substratelayer.
 3. The method of claim 1 , wherein the substrate includes amaterial selected from the group consisting of silicon,silicon-germanium, silicon-carbide, nickel, and gold.
 4. The method ofclaim 1 further comprising etching a portion of the second insulatinglayer and a portion of the substrate layer to release the combstructures.
 5. The method of claim 1 further comprising disposing amasking layer on top of the first conductive layer prior to step b). 6.The method of claim 5 further comprising, prior to said step b),removing selected portions of the masking layer to expose selectedportions of the first conductive layer.
 7. The method of claim 6 whereinstep b) includes etching the exposed portions of the first conductivelayer, the first insulating layer and the second conductive layer. 8.The method of claim 7 , wherein the selected portions are exposed bydisposing a second masking layer over selected portions of the maskinglayer, wherein the second masking layer is resistant to an etch processthat removes portions of the masking layer that are not covered by thesecond masking layer.
 9. The method of claim 8 wherein step b) includesetching the exposed portions of the first conductive layer, the firstinsulating layer and the second conductive layer.
 10. The method ofclaim 9 , further comprising removing selected portions of the secondmasking layer to expose portions of the first conductive layer.
 11. Themethod of claim 10 further comprising etching the exposed portions ofthe first conductive layer.
 12. The method of claim 11 , wherein themulti-layer structure includes a second insulating layer disposedbetween the second conductive layer and a substrate layer.
 13. Themethod of claim 12 wherein the second insulating layer includes amaterial selected from the group consisting of silicon-nitride,silicon-oxide, silicon-carbide, quartz, high resistivity silicon, highresistivity silicon germanium, polyamide, or a polymeric film.
 14. Themethod of claim 12 wherein the substrate layer includes a mterialselected from the group consisting of silicon, silicon-germanium,silicon-carbide, nickel, and gold.
 15. The method of claim 12 , furthercomprising etching one or more portions of substrate layer and thesecond insulating layer.
 16. The method claim 1 , wherein one or more ofthe first and second conductive layers include a material selected fromthe group consisting of silicon-germanium, silicon-carbide, nickel, andgold.
 17. The method of claim 1 , wherein the insulating layer includesa material selected from the group consisting of silicon oxide, siliconnitride, silicon carbide, high resistivity silicon, high resistivitysilicon germanium, quartz, polyamide, and a polymeric film.